Known inhalation devices suitable for the administration to the lung of any inhalation medicament include devices which administer the medicament in liquid form, in dry powder form or as a suspension of the solid medicament in a liquified propellant.
Devices of the first mentioned type include nebuliser devices wherein a fine respirable mist is formed by action of a compressed gas an a sample, by vibration of a piezoelectric crystal or by other ultrasonic means; also, devices of the type described in e.g. International Patent Application WO 91/14468, where the liquid is sprayed through a small aperture.
Devices of the second mentioned type which may provide the medicament in unit dose or multidose form include the well known SPINHALER (Registered Trademark), which is described in UK Patent 1122284, the TURBUHALER Registered Trademark) which is described in U.S. Pat. No. 4,524,769, and the device described in European Patent Application 407028.
Devices of the third mentioned type, which generally contain a pressurised reservoir of liquified propellant containing a suspension of the solid medicament and a metering valve for dispensing a suitable dose, are also very well known in the art and is not necessary to describe any particular type here.
However, it is a general problem with the above devices that the efficiency of administration of an accurate dose of medicament to the lung is severely impaired in the absence of any control of the flow of air through the device. In general, excessive inhalation velocity causes a significant proportion of the dose to impinge on the back of the throat, with a resultant short dose reaching the target area in the lungs. This is known to be a particular problem of devices of the dry powder type which are gaining popularity due to their environmentally friendly attributes.
One way of alleviating this problem in a dry powder device is described in U.S. Pat. No. 5,161,524 (Glaxo) wherein the inhalation device is provided with a secondary air conduit as well as the primary air conduit which provides the path for the inhalation medicament to the lung. If the air flow velocity become too great through the primary air conduit, then the secondary air conduit opens further thus decreasing the air velocity in the primary air conduit.
However, this arrangement suffers from the disadvantage that whilst the velocity of air in the primary conduit may be reduced, a large volume of non drug-containing air is drawn in through the secondary air conduit, with the result that the breath of air necessary to secure a proper dose can become very long and drawn out. Furthermore, the arrangement may not be suitable for all the types of inhalation device described above.